This invention relates to the measurement of quantities of an electroactive species in solution by controlledpotential (C.P.) coulometric analysis.
C.P. coulometry is a method of measuring the quantity of a particular electroactive species in a solution by carrying out an electrochemical reaction involving the electroactive species to be measured. An electroactive species is defined as a species that will undergo a chemical reaction in solution at the surface of an electrode in response to an applied voltage. The reaction chosen must involve the passage of an electric current and knowledge of the oxidation states of the reactants. The amount of current that flows while the reaction proceeds to a determinable fraction of completion provides a measure of the quantity of the substance in solution. This method is well known. It is normally carried out using a C.P. coulometer which is a device for controlling the potential of a working electrode to a selected potential with respect to a reference electrode by applying enough voltage and passing enough current between the working electrode and a counter electrode to cause this selected potential to be maintained. The value of the control potential is selected to favor the particular reaction that is desired and thus to discriminate against unwanted reactions. The coulometer has an integrator module which integrates the current passing between the working and counter electrodes. The integrated current is directly proportional to the amount of electroactive species electrolyzed. When the desired reaction is allowed to go to an exactly known fraction of completion, the number of electron units of charge divided by the fraction electrolyzed and divided by the number of electrons involved in each electrochemical reaction that takes place is equal to the number of molecules of the test substance in the solution.
Two problems generally arise in the making of precise and accurate measurements by C.P. coulometry. The first is the fact that to permit the reaction to proceed substantially to completion often takes an appreciable amount of time. This is especially of concern when many samples are to be analyzed. The time also leads to a second problem in that the longer the time for analysis, the greater is the chance for changes in parameters such as voltage or temperature that may produce error in the readings. One alternative that has been applied to increase the speed of obtaining results in coulometry is to use one of several methods for predicting the end point of the reaction. If this is done without a computer, however, the time saved is taken up in calculation and the cost of using a computer for such predictive end-point analysis is sufficiently high that it is desirable to look for different methods. One such alternative is referred to as an empirical end-point method. This refers to a technique in which the analysis is terminated at what is believed to be a predetermined fraction of the final value. Various methods of stirring the solution of facilitate reaction have also reduced the time for analysis. However, all of the methods of coulometry previously used in which the reaction is not carried to completion have possibilities for error that are intolerable for highly accurate quantitative measurement of substances such as plutonium in solution. Such measurements must be made with acceptable speed and to high accuracy and precision.
It is an object of the present invention to provide a better apparatus for controlled-potential coulometric analysis.
It is a further object of the present invention to provide for controlled-potential coulometric analysis a digital integrator that gives minimal change in net output with changes in temperature of the operating environment, and is stable electronically in use over long periods of time.
It is a further object of the present invention to provide an apparatus for controlled-potential coulometric analysis including a digital integrator that is not subject to the systematic errors found in state-of-the-art analog integrators caused by capacitor leakage, capacitor dielectric absorption, amplifier drift, and amplifier offset.
It is a further object of the present invention to provide for controlled-potential coulometric analysis an integrator that can be calibrated accurately in a straight-forward manner by electrical means to relate chemical equivalents to the electrochemical constant, the faraday.
It is a further object of the present invention to provide an apparatus for controlled-potential coulometric analysis that gives rapid, highly precise and highly accurate results.
It is a further object of the present invention to provide a digital integrator compatable with automation of controlled-potential coulometry.
Other objects will become apparent in the course of the detailed description of the invention.